As a matter of definition, an "optical image bar" comprises an array of optical picture element ("pixel") generators for converting a spatial pattern, which usually is represented by the information content of electrical input signals, into a corresponding optical intensity profile. If the spatial pattern is superimposed on polarized optical radiation, the image bar is "polarized". Although there are a variety of applications for these image bars in a number of different fields, a significant portion of the effort and expense that have been devoted to their development has been directed toward their application to electrophotographic printing, where they may prove to be a relatively low cost and reliable alternative to the flying spot raster scanners which have dominated that field since its inception. Optical displays may also benefit from the use of such image bars, although their application to that field is a secondary consideration at the present time. Some of the more interesting image bar proposals relate to EO TIR (electrooptic total internal reflection) spatial light modulators. In keeping with the teachings of a commonly assigned U.S. Pat. No. 4,396,252 of W. D. Turner, which issued Aug. 2, 1983 on "Proximity Coupled Electro-Optic Devices," an EO TIR spatial light modulator characteristically comprises a plurality of laterally separated, individually addressable electrodes which are maintained on or closely adjacent a reflective surface of an optically transparent electrooptic element, such as a lithium niobate (LiNbO.sub.3) crystal. In operation, substantially the full width of the electrooptic element of such a modulator is illuminated by a linearly polarized, transversely collimated light beam. Thus, when voltages representing the pixels of a linear pixel pattern (e.g., the pixels for a given line of an image) are applied to its individually addressable electrodes, the modulator spatially phase modulates the wavefront of the light beam in accordance with the applied pixel pattern. As a general rule, of course, the spatial wavefront modulation varies as a function of time in accordance with the pixel patterns for successive lines of a two dimensional image, thereby providing a line-by-line representation of the image.
For image bar applications of EO TIR spatial light modulators, prior proposals typically have incorporated Schlieren imaging optics for imaging the modulator onto its output image plane. The frequency plane filtering of a Schlieren imaging system effectively transforms the spatially modulated output radiation of the modulator into a series of correspondingly modulated intensity profiles, but there are embodiments in which a polarization analyzer may be used alone or in combination with a Schlieren stop to read out the spatial modulation produced by the modulator. Thus, as used herein, the phrase "electrooptic image bar" applies to all image bars which embody electrooptic spatial light modulators, regardless of whether the modulators are read out by spatial filtering and/or by polarization filtering.
There have been several significant developments which have reduced the cost and increased the reliability of EO TIR spatial light modulators. Among the improvements that are of particular relevance to image bar applications of these modulators are a "differential encoding" technique that is described in a commonly assigned U.S. Pat. No. 4,450,459 of W. D. Turner et al., which issued May 22, 1984 on "Differential Encoding for Fringe Field Responsive Electro-Optic Line Printers," and an electrical interconnect strategy that is described in a commonly assigned U. S. Pat. No. 4,367,925 of R. A. Sprague et al., which issued Jan. 11, 1983 on "Integrated Electronics for Proximity Coupled Electro-Optic Devices." Briefly, it has been shown that the number of electrodes which such a modulator requires, when used in an image bar having a predetermined resolution, may be reduced by a factor of almost two if the input data samples are differentially encoded on a line-by-line basis prior to being applied to the modulator. Furthermore, it has been demonstrated that more or less conventional VLSI circuit technology can be employed to integrate the modulator electrodes with their addressing and drive electronics, thereby facilitating the orderly and reliable distribution of data samples to the relatively large number of electrodes which customarily are required for reasonably high resolution printing. Electrooptic image bars intrinsically are spatially coherent devices. Axially illuminated EO TIR electrooptic spatial light modulators (i.e., those wherein the incident radiation propogates in a direction that is essentially parallel to the optical axis of the modulator) are especially well suited for use in higher resolution image bars, but they inherently tend to produce interpixel intensity nulls because they spatially modulate the incident radiation by diffractively scattering optical energy into positive and negative diffraction orders which are more or less angularly symmetrical about a zero order or unmodulated component. These positive and negative diffraction orders (collectively referred to herein as "higher order diffraction components") define the upper and lower spatial frequency sidebands, respectively, of the modulated radiation, so they coherently contribute to the effective spatial modulation bandwidth of the modulator, provided that their relative phase is preserved. Unfortunately, however, whenever such spatially coherent radiation is brought to focus to form an image, adjacent pixels of opposite phase destructively interfere with each other, thereby producing undesireable interpixel intensity nulls. For example, differential encoding produces adjacent pixels of opposite phase.
Others who have attempted to develop essentially null-free image bars embodying axially illuminated EO TIR spatial light modulators have recognized that the unwanted interpixel intensity nulls are caused by destructive interference, so their work is most interesting. As described in a commonly assigned U.S. Pat. No. 4,437,106 or R. A. Sprague, which issued Mar. 13, 1984 on "Method and Means for Reducing illumination Nulls in Electro-Optic Line Printers," one of the prior null suppression proposals suggests scattering light into the null regions in accordance with a pattern having an angular orientation and/or a spatial frequency which tends to inhibit the ability of the unaided eye to resolve the nulls, even when the imaging is performed at normal exposure levels. This approach preserves the internal spatial coherency of the output radiation of the image bar (i. e., the relative phase of its positive and negative diffraction orders), while reducing the observable affects of the nulls. Another commonly assigned U.S. Pat. No. 4,483,596 of S. W. Marshall, which issued Nov. 20, 1984 on "Interface Suppression Apparatus and Means for a Linear Modulator," describes an alternative approach pursuant to which a polarization retardation plate or the like is provided for orthogonally polarizing the positive and negative diffraction orders of the modulated output radiation of the image bar, thereby preventing them from destructively interfering with each other. That effectively suppresses the interpixel intensity nulls, but it also destroys the relative phase information between the positive and negative diffraction orders, thereby reducing the effective spatial bandwidth of the image bar by a factor of two. Moreover, it may be relatively diffilcult and expensive to take full advantage of this orthogonal polarization concept in practice because of the wide range of incident angles at which light from different points along an image bar of appreciable width would fall on the polarization retardation plate.
A copending and commonly assigned United States patent application of D. L. Hecht, which was filed Dec. 13, 1985 under Ser. No. 808,709, now U.S. Pat. No. 4,673,953, on "Null Supression for Optical Image Bars" provides a null suppression technique which is believed to be superior to the others, although it initially appeared to be of limited utility because it was developed for polychromatic image bars and relied upon chromatic dispersion for decomposing the spatially modulated output radiation of such an image bar into non-interfering (i.e., mutually orthogonal) and spatially displaced components. As will be appreciated, the wavelength spread that is required is a significant limitation because many image bars, including those which embody laser illuminated EO TIR spatial light modulators, are monochromatic. However, the basic concept of decomposing the spatially modulated output radiation of a coherent image bar into mutually orthogonal components not only preserves the internal spatial coherency of the output radiation of the image bar, but it also is instrumental in producing laterally offset and redundantly modulated intensity profiles which spatially sum with each other on an intensity basis to produce an essentially null-free image having relatively uniformly shaped pixels. The challenge, therefore, is to provide a suitable mechanism for applying such a null suppression technique to monochromatic image bars.